Module structure and module comprising it

ABSTRACT

A module is provided that has high reliability in the junction between a ceramic circuit board and a heat sink, undergoes small changes in shape and warp of a module structure comprising a ceramic circuit board and a metal heat sink event under a temperature history during power module assembly and power module actual use, eliminates an unfavorableness during power module assembly, and maintains high reliability over a long period of time. A module structure characterized in that in a module member formed by bonding the ceramic circuit board to the metal heat sink via a metal plate (A) the main component of which is aluminum, the metal plate (A) has a thickness of 400 μm or more and 1,200 μm or less.

TECHNICAL FIELD

The present invention relates to a module comprising a metal heat sinkand a ceramic circuit board on which is mounted a heat-generatingelectric component, such as a power device. The present inventionparticularly relates to a module which is to be suitably used for powersource and to a module structure used in the module.

BACKGROUND ART

In recent years, due to the development of power electronics,apparatuses controlled by a power device, such as IGBT and MOS-FET arerapidly increasing. In particular, power-device controlling is rapidlyproceeding in transportation apparatuses such as electric railwaytransportation apparatus and vehicles. Furthermore, in accordance withincreased concern about environmental problems, for example, electriccars, and hybrid cars using both a gasoline engine and an electricmotor, have begun to be placed on the market, so that the demand forpower modules to be mounted thereon is expected to increase. For suchuse, particularly high reliability is required in view of the object ofits use.

In a conventional power module, in order to prevent the temperature of asemiconductor device from raising above a predetermined temperature bycausing the heat generated in the semiconductor or the like to dissipateoutside, the structure thereof is generally such that a semiconductordevice is mounted on a ceramic circuit board of aluminum oxide (Al₂O₃),silicon nitride (Si₃N₄), aluminum nitride (AlN), or the like bysoldering, and is soldered to a heat sink which is composed of a metalsuch as copper (Cu) or aluminum (Al).

In the case of such a structure, however, it may occur that a solderlayer between the ceramic circuit board and the heat sink cracks whensubjected to a heat cycle during the operation of the semiconductordevice, or to changes in the temperature of the operating environment,or the like. The solder layer cracks due to a thermal stress generatedby a difference between the thermal expansion of a ceramic substrate andthat of the heat sink. The presence of cracks in the solder layer(hereinafter simply referred to as the “solder cracks”) lowers thedissipation of the heat generated in the semiconductor device andelevates the temperature of the semiconductor device. As a result, thesemiconductor device becomes deteriorated, and the reliability of thepower module is lowered as a whole.

As semiconductor apparatus becomes highly integrated and requires higherpower, higher heat dissipation is demanded, and it is desired to makethe solder free of lead in view of the environmental pollution. However,a so-called lead-free solder has a problem that the reliability thereofis lower than that of Pb—Sn solders which are currently used widely,although the heat conductivity of the lead-free solider is high.

In order to avoid these problems, it is being studied to use in the heatsink an Al—SiC composite material or a Cu—Mo composite material, whichhas such a thermal expansion coefficient that is closer to that of theceramic substrate. However, in comparison with a conventional metal heatsink, such a heat sink has the problems that the heat sink has to befabricated by a special process and the processing step and the surfacetreatment step therefor are more costly and such a heat sink is by farmore expensive.

On the other hand, there has been conducted a trial of directly bondinga heat sink and a ceramic circuit board by using a brazing filler metalinstead of a solder in order to avoid the occurrence of solder crackingand to improve on heat dissipation (refer to JP-A-9-97865 andJP-A-10-270596).

In this case, however, such problems are caused that debonding of ajunction interface and cracking of the ceramic circuit board become aptto occur due to the thermal stress generated by a difference in thermalexpansion between the ceramic circuit board and the metal heat sink, andthat the stress applied to the solder that bonds a semiconductor deviceand the ceramic circuit board is increased, so that cracking of thesolder under the semiconductor device becomes apt to occur more easily.Furthermore, the shape and warp of the heat sink may greatly changeunder a heat history during the assembly process of the power module orunder the actual use thereof, so that there may be a case where aproblem is caused during the assembling of the power module, and thelowering of heat dissipation performance takes place due to a decreasein the close contact between the heat sink and a heat dissipation block.

The present invention was made in view of the above-mentionedcircumstances, and an object of the present invention is to provide amodule which has a module structure comprising a ceramic circuit boardand a metal heat sink, the module structure being such that the changesin the shape thereof are small even under a heat history during theassembly thereof and under the actual use thereof, and the assemblingthereof is easy, abnormalities such as the debonding at the junctioninterface, the cracking of a ceramic substrate, the formation of cracksin a solder layer, and the like, are difficult to occur, with excellentheat dissipation performance, and with high reliability that can bemaintained over a long period of time.

DISCLOSURE OF INVENTION

The inventors of the present invention have conducted variousexperimental studies in order to achieve the above-mentioned object andmade the present invention by finding that in a structure provided witha stress buffer layer between a ceramic substrate and a metal heat sink,when various countermeasures are taken to the metal heat sink and ametal plate serving as the stress buffer layer, the module structureobtained undergoes small changes in shape and warp even under a heathistory during the assembly and actual use thereof, without the heatdissipation performance thereof being impaired.

Specifically, the present invention provides a module structure in whicha ceramic circuit board is bonded to a metal heat sink via a metal plate(A) the main component of which is aluminum, characterized in that themetal plate (A), with the main component thereof being aluminum, has athickness of 400 μm or more and 1200 μm or less.

The module structure is preferably a module structure characterized inthat the above-mentioned metal heat sink is composed of an aluminumalloy having a Vickers hardness of 30 Hv or more after being subjectedto heat treatment at 630° C. for 4 minutes.

The above-mentioned metal plate (A) is preferably bonded to the ceramiccircuit board and to the metal heat sink via a brazing filler metal.When as the brazing filler metal, there is used such a brazing fillermetal that contains Al as the main component, Mg, and at least oneelement selected from the group consisting of Cu, Zn, Ge, Si, Sn and Ag,highly reliable bonding can be obtained.

The module is furthermore preferably a module which is characterized bycomprising the above-mentioned module structure, a heat-generatingelectrical component which is mounted on a desired position on acircuit-formed metal plate (B) which is disposed on a side opposite themetal plate (A) of the ceramic circuit board, and a notch provided onthe surface of the above-mentioned metal plate (A) and/or theabove-mentioned metal heat sink, wherein when a cross section of themodule is assumed, the above-mentioned notch is provided in such aregion that is outside a frustum of cone region formed by a group ofstraight lines drawn downward at an angle of 45° with respect to thevertical direction from the edges of the heat-generating electriccomponent in contact with the metal plate (B).

The notch is preferably provided at the surface of the metal plate (A)on the side in contact with the metal heat sink, or at the surface ofthe metal heat sink covered with the ceramic substrate when viewed fromthe side where the heat-generating electric component of the module isprovided, or at the surface of the metal heat sink which is not coveredwith the ceramic substrate when viewed from the side where theheat-generating electric component of the module is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: a cross-sectional view of a module of Example 12 of the presentinvention.

FIG. 2: a cross-sectional view of a conventionally known module ofComparative Example 4.

FIG. 3: a cross-sectional view of a conventionally known module ofComparative Example 5.

FIG. 4: a cross-sectional view of a module of Example 13 of the presentinvention.

FIG. 5: a cross-sectional view of a module of Example 14 of the presentinvention.

FIG. 6: a cross-sectional view of a module of Example 15 of the presentinvention.

EXPLANATION OF REFERENCE NUMERALS

1: ceramic substrate

2: heat-generating electric component (semiconductor device)

3: metal plate (B)

4: metal plate (A)

5: heat sink

A: a frustum of cone formed by straight lines extending through theanother metal plate and extending from edges of said heat-generatingelectric component in contact with the another metal plate at an angleof 45° with respect to a vertical direction.

B: notch

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be explained.

As a ceramic substrate for use in the present invention, any substratemay be used as long as it satisfies the required characteristics such aselectric insulation, heat conductivity, and mechanical strength. Inparticular, aluminum nitride (AlN), which is a ceramic having a highheat conductivity, and silicon nitride (Si₃N₄), which has a highstrength and a relatively high heat conductivity, are suitable.

A module structure of the present invention has such a structure thatcomprises a metal heat sink and a ceramic circuit board which is bondedto the metal heat sink. Broadly, two techniques are known for reducingthermal stress when materials with different properties are bonded. Inthe first technique, a heat sink with a low thermal expansioncoefficient is used in order to reduce the difference in thermalexpansion between two materials. This method, however, has a problemwith respect to the cost as mentioned previously. The second techniqueis used in the present invention and is based on the idea of absorbingthermal strain by inserting a stress buffer layer between the metal heatsink and the ceramic circuit board, by which technique the relaxation ofthermal stress is carried out by using the plastic deformation of amaterial with a low modulus of elasticity in an intermediate layer(stress relaxation layer).

The present invention is characterized in that as the stress bufferlayer is used a metal plate with the main component thereof beingaluminum, having a thickness of 400 μm or more and 1,200 μm or less. Asmentioned above, it is essential that the stress relaxation layer has asmechanical characteristics a low modulus of elasticity and a low yieldstrength. However, in the present invention, for the applicationthereof, it is necessary that the stress relaxation layer meet suchrequirements that the thermal conductivity thereof be high for heatdissipation, that the stress relaxation layer not melt when asemiconductor device is soldered, and that the stress relaxation layercan be bonded to the ceramic substrate and to the metal heat sink with asufficient strength.

The inventors have conducted an extensive study on various materials andthe thickness thereof in order to find a stress relaxation layer whichsatisfies the above-mentioned requirements, and have made the presentinvention. In the present invention, as the stress relaxation layer, ametal plate with the main component thereof being aluminum is selectedand is used as the above-mentioned metal plate (A). In the metal plate(A), for example, JIS (the Japanese Industrial Standards) named 1000series aluminums are preferable for use, and of such aluminums, a highpurity aluminum with a purity of 99 mass % or more is preferable, and ahigh purity aluminum with a purity of 99.9 mass % or more is morepreferable.

In the present invention, the metal plate (A) has a thickness of 400 μmor more and 1,200 μm or less, preferably a thickness of 600 μm or moreand 1,000 μm or less. When the thickness exceeds 1,200 μm, there may bea case where the stress generated by the metal plate (A) and the heatsink reaches the side of the metal plate (B) and has an adverse effecton the durability of a solder under a silicon chip. Furthermore, thisdegrades pattern accuracy when forming a pattern by etching andincreases the cost, so that the exceeding of the thickness is notpreferable. When the thickness is less than 400 μm, it may occur thatthe function as the buffer layer becomes insufficient due to thehardening thereof caused by the diffusion of elements from the junction,and that an Al buffer layer cannot withstand repeated stressing causedby heat cycle and is ruptured.

The present invention is characterized in that the reliability of thepower module can be secured even though the metal heat sink is made ofcopper, aluminum, or an alloy comprising copper or aluminum as the maincomponent, which is inexpensive and has high heat conductivity. Analuminum alloy is suitable for use in the heat sink since it islightweight and inexpensive. When an aluminum alloy is used, preferableis such an aluminum alloy that has a Vickers hardness of 30 Hv or more,more preferable is such an aluminum alloy that has 60 Hv or more, afterbeing subjected to heat treatment at 630° C. for 4 minutes.

The use of an aluminum alloy having the above-mentioned features for thefabrication of the heat sink can make extremely small the warping of theobtained module structure in collaboration with the fact that thethickness of the above-mentioned metal plate (A) is in an appropriaterange. Furthermore, the module structure undergoes small changes inshape and warp under a heat history during the power module assembly andpower module actual use, and there can be prevented the debonding of thejunction interface and the rupture of the metal plate (A) serving as thebuffer layer, so that the breaking of the semiconductor device and theelevation of the thermal resistance can be prevented, which have goodeffects on each kind of reliability of the power module.

Any aluminum alloy can be used in the heat sink as long as the aluminumalloy has the above-mentioned characteristics. Examples of such analuminum alloy include aluminum alloys containing Al and at least one ofSi or Mg in an appropriate amount, for instance, aluminum alloys of JISnamed 2000 series, 5000 series, 6000 series, and 7000 series.

The content of Si and Mg in the aluminum alloy is preferably about 0.1to 4.0 mass % from the viewpoint of physical properties and workability.Even if the content of Si and Mg exceeds the above range, as long as theVickers hardness thereof is 30 Hv or more, more preferably 60 Hv ormore, the above aluminum alloys are excellent in that the changes inshape and warp of the module structure can be made small, which is oneof features of the module structure of the present invention.

The aluminum alloy for use in the present invention may contain othercomponents and impurities as long as the above-mentioned characteristicsare satisfied. Aluminum alloys containing Mg, Cu or Zn in an amount of2.0 mass % or more have high Vickers hardness and high bending strengthand are excellent in that the obtained module structure undergoes smallchanges in shape and in warp. Furthermore, it is only necessary that theframe of the heat sink material be composed of the above-mentionedaluminum alloy, and it is not necessary that the entire heat sink becomposed of the above-mentioned aluminum alloy.

The Vickers hardness of the aluminum alloy suitable for the heat sink ofthe present invention has been shown. However, the property of thealuminum alloy can also be indicated in terms of the load anddisplacement by measuring the bending strength thereof after hightemperature annealing of the aluminum alloy. Specifically, the sameeffect as mentioned above can be obtained in the case where a 5 mmthick, 5 mm wide test piece is heated at 600° C. for 10 minutes, andthen the three-point bending strength is measured with a 30 mm span,when the displacement becomes 200 μm, the load is 200 N or more,preferably 300 N or more length.

The heat sink may be in the shape of a plate with a flat back surface orwith fins being formed. The heat sink may have such a structure that acooling medium can pass through the inside of the heat sink. Such astructure is preferable since a power control unit including the powermodule and a cooler can be reduced in size in its entirety, whichcontributes to the reduction of the cost thereof.

Furthermore, in the module structure of the present invention, theabove-mentioned metal plate (A) is preferably bonded to the metal heatsink via a brazing filler metal or a lead-free solder from theviewpoints of the quality of the material, the shape, the workability,and the bonding strength of the module structure.

When a Pb—Sn type solder is used for bonding the heat sink and theceramic circuit board, there are caused the above-mentioned problemssuch as the lowering of the reliability caused by solder cracks in thecourse of the heat cycle, the environmental problems, the necessity forplating the surface of aluminum due to the poor solder wettability ofaluminum, so that the use of the Pb—Sn type solder is not preferable.However, these problems can be overcome by using a brazing filler metal.Furthermore, in the present invention, it is furthermore preferable thatthe metal plate (A) the main component of which is aluminum be bonded tothe ceramic circuit board via a brazing filler metal.

In the present invention, the brazing filler metal for bonding the heatsink and the metal plate (A) and for bonding the metal plate (A) and theceramic substrate may be appropriately selected according to the kind ofmetal heat sink to be used, or the like. In particular, when there isused a brazing filler metal comprising Al as the main component, Mg, andat least one element selected from the group consisting of Cu, Zn, Ge,Si, Sn and Ag, bonding with high reliability can be obtained. It isappropriate that the content of Mg in the above-mentioned brazing fillermetal is 0.1 to 2.0 mass %. When the content is less than 0.1 mass %, asufficient junction cannot be obtained, while when the content is morethan 2.0 mass %, there may be a case where the thermal shock resistanceof the junction is lowered or things unfavorable occur for the operationof a bonding furnace. As aluminum alloys suitable for use in the brazingfiller metal of the present invention, there can be used, for example,aluminums with appropriate compositions in JIS named 2000 series, 3000series, 5000 series, 6000 series, and 7000 series.

The brazing filler metal may be either an alloy or a non-alloy, whichmay be in any form of foil, powder, mixed powder, or mixed powdercontaining a compound in which the above-mentioned metal componentremains at a temperature not more than the bonding temperature, or anycombination thereof. An alloy foil is excellent, for instance, in thatthe junction has heat cycle resistance, and that micro voids are hardlyformed, and that it is easy to handle. In particular, a JIS named 2017aluminum alloy foil is good for bonding the metal plate (A) and theceramic substrate.

Furthermore, in order to bond the heat sink and the metal plate (A), itis necessary to set the bonding temperature at a temperature not higherthan the melting point of the heat sink, so that in order to lower themelting point of the above-mentioned brazing filler metal composition,if necessary, the amount of the components other than Al may beincreased. For example, the most preferable result can be obtained whenthe above-mentioned Al alloy foil and a silver foil or a silver powderare used in combination. With respect to the thickness of the brazingfiller metal, when the thickness is 10 to 60 μm, preferably 10 to 40 μm,good reproducibility and strong junction with excellent heat cycleresistance performance can be obtained.

When the Al alloy heat sink and the metal plate (A) the main componentof which is aluminum are bonded, generally the bonding is preferablycarried out in vacuum with the application of heat thereto. At thistime, when both junction surfaces have large irregularity or roughness,there may be a case where bonding failure frequently occurs or the heatcycle resistance of the junction becomes inferior. When bonding isperformed in nitrogen, the shape of the surface of the component to bebonded is largely affected, and bonding failure is apt to occur at anouter peripheral portion of the metal plate (A). In particular, when anextrusion material is used for the heat sink, there is a case whereextrusion marks formed on the surface of the heat sink causes bondingfailure or debonding during a heat cycle test. These problems can beresolved by use of a Mg-containing Al alloy foil or a combination of analloy powder and a Ag powder or a Ag foil. The use of such an alloy andAg in combination is epoch-making in that a junction having sufficientdurability and high reliability can be obtained even in a nitrogenatmosphere. Moreover, even when the Vickers hardness of the heat sinkafter the heat treatment thereof is high or when the bending strengththereof is high, the use of the combination is preferable since a strongjunction which is difficult to be unbonded can be obtained. Since thismethod makes it possible to perform junction in nitrogen, the junctioncan be carried out in a common continuous furnace with a nitrogenatmosphere, so that the cost for the production can be sharply reduced.

In performing brazing, when the brazing filler metal is an alloy foil,the brazing is carried out by holding the alloy foil between the heatsink and the metal plate (A) or between the metal plate (A) and theceramic substrate, with the application of heat thereto in vacuum, innitrogen, or in an inert gas. When an alloy powder or a metal powdermixture is used as the brazing filler metal for bonding the heat sinkand the metal plate (A) or for bonding the metal plate (A) and theceramic substrate, the bonding can be carried out by coating the alloypowder or the metal powder mixture onto one of the surfaces thereof byuse of a roll coater or a screen printing machine. When the amount ofcoating is insufficient, sufficient bonding cannot be carried out, whilewhen the amount of coating is excessive, the brazing filler metaldisadvantageously flows out beyond the junction, or a hard, fragilelayer is formed at the interface so that the reliability of junction maybe impaired. The amount of coating is preferably about 1 to 5 mg/cm².

When the heat sink and the metal plate (A) are bonded by use of the Agalloy brazing filler metal foil and a Ag powder in combination, theeffect of using silver in combination can be obtained by merely coatingthe silver powder onto one face of the brazing filler metal foil, theheat sink or the metal plate (A). The amount of coating of the silverpowder is sufficient in the amount of about 1 to 3 mg/cm².

Furthermore, in the present invention, a lead-free solder may be used tobond the metal heat sink and the metal plate (A). It is said that incomparison with Pb—Sn type solders, lead-free solders are harder and donot readily undergo plastic deformation, so that usually cracks arereadily formed in lead-free solders by heat cycle. However, in thisinvention, a sufficiently reliable module structure can be obtained. Thereliability can be secured by using as the lead-free solder a Sn—Ag—Cutype solder or a Sn—Zn type solder. In particular, a Sn—Ag (3 mass %)—Cu (0.5 mass %) solder is preferable to use.

The metal plate (B), which is disposed on one face of the ceramicsubstrate and on a portion of which a heat-generating electricalcomponent is mounted to constitute a circuit, may be composed of anymetal having good electric conductivity. As such a metal, copper,aluminum, and alloys thereof are preferably employed, which areinexpensive and have high heat conductivity. As the above-mentionedcopper and aluminum, preferably employed are those having high puritywhich exhibit high electric conductivity and high plastic deformationperformance under the generation of stress.

In the present invention, it is preferable to provide a notch at aparticular position of an upper surface and/or a lower surface of themetal plate (A) and/or the metal heat sink, which metal plate (A) isdisposed on the ceramic substrate on the side of the heat sink so as tobe in contact with the heat sink. This is because the notch is capableof relaxing the strain of the metal plate (A) which is caused by thermalstress generated by a difference between the thermal expansioncoefficient of the ceramic substrate and that of the heat sink duringthe heat treatment, for instance, at the time of mounting aheat-generating electric component such as a semiconductor device, whilemaintaining almost the same level of heat dissipation performance as inthe case where no notch is formed, and the notch is also capable ofreducing the deformation of this module structure under a temperaturehistory thereof.

In the present invention, the position for introducing the notch, namelythe place where the notch is provided, is set, when a cross section ofthe module is assumed as shown in FIG. 1 to FIG. 6, outside the regionof a frustum of cone formed by a group of straight lines drawn downwardat an angle of 45° with respect to the vertical direction from the edgesof the heat-generating electric component in contact with the metalplate (B), that is, outside the region of the frustum. By forming thenotch in the above-mentioned specific position, the dissipationperformance of the heat generated from electric components such as thesemiconductor device in the module or from circuits, is not impaired, sothat the occurrence of such phenomena as the malfunction of thesemiconductor device and the shortening of the life can be prevented,without elevating the temperature of the semiconductor device.

The position for introducing the notch can be varied in accordance withthe size, the shape and the mounting position of the heat-generatingelectric component to be mounted. For the reduction of the deformationof a heat dissipation structure, the most effective way is to provide anotch introduced from the side of the heat sink into the metal plate (A)which is provided so as to come into contact with the heat sink on theheat sink side of the ceramic substrate. The greater the depth of thenotch, the greater the effect of reducing the deformation of the modulestructure. It is preferable that the metal plate be divided, but it isnot always necessary to divide the metal plate. The width, the shape,and the number of the notches may be suitably selected unless the notchis in a region where the heat dissipation performance is impaired.

Furthermore, by introducing the notch into a portion of the heat sinkwhere the heat sink is in contact with the metal plate (A), the sameeffect can be obtained. In this case, the notch can be introduced intothe surface of the heat sink by a simple groove formation working or thelike, which is effective for attaining excellent productivity. Also inthis case, the greater the depth of the notch, the greater the effect.However, the depth is preferably not more than ½ the thickness of theheat sink. When there is provided a notch which is deeper than that,there may be a case where the module structure, which is fabricated bybonding the ceramic circuit board and the heat sink, is greatlydeformed. The width, the shape, and the number of the notches may besuitably selected as long as the notches are within the above-mentionedspecific range.

In the case where no notch can be provided at the interface between themetal plate and heat sink due to the restrictions thereon imposed by thesize, the shape, and the position of the heat-generating electriccomponent, the deformation reducing effect can be obtained by providingthe notch at an upper surface and/or a lower surface of the heat sinkother than the junction interface. In this case, greater is the freedomto select a place for the notch, which is spatially remote from portionswhich perform important functions of the module, where theheat-generating electric component, circuits, the ceramic substrate, andothers are located, so that the productivity is excellent and as aresult, there can be obtained the effect of making the cost of themodule furthermore inexpensive.

The ceramic circuit board for use in the present invention can be easilyproduced by bonding a ceramic to the metal plate (B) for circuits, andthe metal plate (A) the main component of which is aluminum, forming acircuit by a conventionally known technique, such as etching ormachining, or forming a notch in the metal plate (A). Alternatively, theceramic circuit board can also be produced by bonding a metal plate (A,B) on which circuits and a notch have been mounted in advance to aceramic substrate.

To a method of obtaining the module structure and the module of theinvention, a conventionally known method can be applied. A method whichwill be described later has good reproducibility, and high productivity,by which the module structure and the module of the present inventioncan be obtained.

Specifically, preferable is a method by which a ceramic circuit board ismade ready in advance, the ceramic circuit board being provided inadvance by brazing or the like with a circuit-formed metal plate (B) ona front surface thereof and a metal plate (A) the main component ofwhich is aluminum, serving as a stress relaxation layer, on a backsurface thereof, a brazing filler metal is disposed between the surfaceof the stress relaxation layer and a metal heat sink, and the ceramiccircuit board and the metal heat sink are bonded with the application ofheat thereto under pressure; alternatively, preferable is a method bywhich the metal plate (B) for circuit, the ceramic substrate, the metalplate (A) the main composition of which is aluminum, serving as a stressrelaxation layer, and the metal heat sink are successively stacked, witha brazing filler metal being interposed between each of them, and arebonded simultaneously.

Furthermore, in order to fabricate a module structure provided with anotch, preferable is a method by which a notch is introduced in advanceinto the surface of the metal plate (A) of the ceramic circuit board orinto the surface of the metal heat sink by etching, machining or thelike, then disposing a brazing filler metal between the metal plate (A)of the ceramic circuit board and the metal heat sink, and bonding theceramic circuit board and the metal heat sink with the application ofheat thereto under pressure; alternatively, a method by which the metalplate (B) for circuit, the ceramic substrate, the notch-introduced metalplate (A) the main component of which is aluminum, and the metal heatsink are successively stacked, with a brazing filler metal beinginterposed between each of them, and are bonded simultaneously.Furthermore, in the latter method, the metal plate (B) for circuit maybe one with a circuit being formed in advance, or a circuit may beformed after the bonding, with the application of a method such asetching. In the case where the notch is introduced into the region otherthan the injunction interface of the heat sink, the notch may beprovided after the bonding.

Next, by mounting an electronic component, such as a semiconductordevice, by soldering or the like onto the circuit of the modulestructure that is composed of the ceramic circuit board and the metalheat sink, and when necessary, by conducting wire bonding or like tocomplete a circuit, the module of the present invention can be obtained.

The power module on which a high heat-generating electric component suchas a high output semiconductor device, assembled by use of the modulestructure of the present invention, is used with the attachment of aheat dissipation unit such as heat dissipation fins through a high heatconductive grease when the metal heat sink is a solid plate. When theheat sink is in a shape with the attachment of fins thereto, the heatsink is used as it is. When the heat sink is a pipe through which acooling medium is allowed to flow, pipe installation for allowing thecooling medium to flow is carried out and then the power module is used.

The present invention will now be explained more specifically by givingExamples and Comparative Examples below. The present invention is notlimited by these Examples.

EXAMPLES 1, 2, 3, 4, 5 AND COMPARATIVE EXAMPLES 1, 2

As a ceramic substrate was prepared a silicon nitride substrate with aheat conductivity of 75 W/mK by the laser flash technique and an averagethree-point bending strength of 650 Mpa, with a size of 34×34×0.635 mm.An Al plate with a purity of 99.99% and a thickness of 0.4 mm(hereinafter referred to as the Al circuit plate) was prepared as themetal plate for the circuit. As an Al plate for a stress buffer layer(hereinafter referred to as the buffer Al plate) there were prepared Alplates with a purity of 99.99% and various thicknesses as shown in Table1.

On the front side and the back side of the silicon nitride substrate,the above-mentioned Al circuit plate and the buffer layer Al plate wereoverlaid through a JIS named 2017 Al foil (thickness of 20 μm), and apressure of 5 MPa was applied thereto in the vertical direction. Both ofthe above-mentioned Al plates were bonded to the silicon nitridesubstrate in a vacuum of the order of 10⁻³ Pa, with the application ofheat thereto at 635° C.

After the bonding, an etching resist was applied by screen-printing to adesired portion of the surface of the Al plate, and a circuit patternwas formed by etching the portion with a ferric chloride solution,whereby a ceramic circuit board was fabricated.

A JIS named 1050 aluminum plate with a size of 50×50×4 mm was preparedas the heat sink. A JIS named 2017 aluminum foil with a thickness of 20μm was held between the buffer Al plate bonded to the above-mentionedceramic board and the heat sink, and was subjected to heat treatment invacuum on the order of 10⁻³ Pa at 600° C. for 4 minutes, with theapplication of a pressure of 5 MPa thereto in the vertical directionusing a graphite jig, whereby the ceramic circuit board was bonded tothe heat sink. The aluminum surface of the bonded product was platedwith nickel by electroless plating to obtain a module structure. Tenmodule structures with the same structure were fabricated.

A 10×10×0.3 mm silicon chip with a Au plated back side was bonded to theAl circuit surface of the module structure fabricated using a solderwith the mass ratio of lead to tin being 90:10 at 350° C.

The module was subjected to 3,000 heat cycles with each cycle being −40°C.×30 minutes→room temperature×10 minutes→125° C.×30 minutes→roomtemperature×10 minutes. Subsequently, the interface between the ceramicsubstrate and the heat sink and the interface between the ceramicsubstrate and the silicon chip were examined with an ultrasonic flawdetector to determine the presence or absence of cracks and debonding atthe bonding interfaces. The results are shown in Table 1.

Except in Comparative Example 1, debonding of the bonded portions andcracks in the silicon nitride substrate, caused by the heat cycle, werenot observed in any of the modules. In Comparative Example 1, part ofthe bonded portion became debonded, and fatigue breaking of the bufferAl plate conspicuously took place. In Example 1, fatigue breaking wasobserved at the corners of the buffer layer Al plate. In ComparativeExample 2, cracks were observed over the entire surface of the solderlayer between the silicon chip and the circuit board. In Example 5, somecracks were observed in the solder layer.

TABLE 1 Thickness of buffer Al plate Ratio of defects after (μm) 3,000heat cycles (*) Example 1 400  4/10 Example 2 600  1/10 Example 3 800 0/10 Example 4 1000  1/10 Example 5 1200  3/10 Comparative 200 10/10Example 1 Comparative 1600  8/10 Example 2 (*) Number of defective testpieces to the total number of test pieces

EXAMPLES 6, 7, 8, 9, 10, 11 AND COMPARATIVE EXAMPLE 3

By using 7 types of heat sinks shown in Table 2, modules were preparedand evaluated with a repeating number of 10, to make them Examples ofthe present invention and Comparative Examples.

As a ceramic substrate was prepared an AlN (aluminum nitride) substratewith a size of 34×34×0.635 mm, a heat conductivity of 180 W/mK by thelaser flash method, and an average three-point bending strength of 400Mpa. 2 JIS named 1085 Al (aluminum) plates, each having a size of30×30×0.4 mm, were prepared, one being a metal plate to be made acircuit, and the other being a metal plate to be provided on the surfaceof the above-mentioned AlN substrate directed toward the heat sink(hereinafter referred to as the substrate back surface).

The above-mentioned Al plates were overlaid on the above-mentioned AlNsubstrate, one on the front surface of the AlN substrate and the otheron the back surface of the AlN substrate, each of the Al plates beingsuperimposed through a JIS named 2017 Al foil (20 μm thick), and apressure of 10 Mpa was applied thereto in the vertical direction. The Alplates and the AlN substrate were bonded in a vacuum of 10⁻³ Pa with theapplication of heat thereto at 630° C. for 20 minutes. After thebonding, an etching resist was applied by screen-printing on a desiredportion of the surface of the Al plate, and a circuit pattern was formedby etching with a ferric chloride solution, whereby a ceramic circuitboard was fabricated.

Next, as heat sinks were prepared aluminum plates with a size of 46×46×4mm, having compositions as shown in Table 2. Silver powder was appliedby screen-printing in an amount of 1.5 mg/cm² onto theheat-sink-contacting surface of each ceramic circuit board. A JIS named2017 Al foil with a thickness of 20 μm was interposed between theapplied silver powder and the heat sink, and the heat sink and theceramic circuit board were bonded by heat treatment at 510 to 600° C. ina nitrogen atmosphere for 4 minutes, with the application of a pressureof 10 MPa in the vertical direction by use of a graphite jig. Finally,the entire surfaces of the substrate and a heat dissipation plate weresubjected to Ni electroless plating, whereby a module structure wasobtained.

A 13×13×0.4 mm silicon chip with an Au plated back side was bonded tothe Al circuit surface of the fabricated module structure by use of asolder with the mass ratio of lead to tin being 90:10 at 350° C.

The warp of the silicon chip of the module obtained by the aboveprocedure was measured. The warp was evaluated as the difference betweenthe height of the center and the height of the opposite ends of thesilicon chip on the diagonal line thereof. The average warp of tensamples is shown in Table 3.

TABLE 2 Composition of heat sink Vickers hardness (mass %) (*) (Hv)Example 6 0.4 Si—0.5 Mg—balance Al 30 Example 7 0.55 Si—0.75 Mg—balanceAl 47 Example 8 0.2 Si—0.4 Mg—3.5 Cu— 71 balance Al Example 9 0.25Si—2.4 Mg—balance Al 68 Example 10 0.4 Si—4.2 Mg—balance Al 92 Example11 0.4 Si—2.1 Mg—5.1 Zn— 83 balance Al Comparative Al (99.99) 13 Example3 (*) After heat treatment at 630° C. for 4 minutes

TABLE 3 Warp (μm) Example 6 5.3 Example 7 4.7 Example 8 2.1 Example 92.1 Example 10 −0.3 Example 11 0.2 Comparative 23.6 Example 3 *Warp isan average of the differences between the height at the opposite endportions and the height at a central portion on a diagonal line of thesilicon chip. +: The side of the silicon chip was convex. −: The side ofsilicon chip was concave.

EXAMPLE 12 AND COMPARATIVE EXAMPLE 4

As a ceramic substrate was prepared a silicon nitride substrate (size34×26×0.635 mm) with a heat conductivity of 70 W/mK measured by thelaser flash method and an average three-point bending strength of 750MPa. As the metal plate (A) and the metal plate (B) were preparedaluminum plates with a purity of 99.99% and a thickness of 0.4 mm.

The aluminum plates were overlaid on the silicon nitride substrate, oneon the front surface of the silicon nitride substrate and the other onthe back surface of the silicon nitride substrate, each of the aluminumplates being overlaid through a JIS named 2017 Al foil (20 μm thick),and a pressure of 5 MPa was applied thereto in the vertical direction.The Al plates were bonded to the silicon nitride substrate in a vacuumon the order of 10⁻³ Pa with the application of heat thereto at 635° C.

After the bonding, an etching resist was applied by screen-printing ontoa desired portion of the surface of the upper and lower aluminum plates,and a circuit pattern was formed on the metal plate (B) by etching witha ferric chloride solution, whereby a ceramic circuit board wasfabricated, with the formation of a notch for the relaxation of strainin the metal plate (A) in Example 12. The notch was provided in such aregion that is outside a frustum of cone region formed by a group ofstraight lines drawn downward at an angle of 45° with respect to thevertical direction from the edges of a semiconductor device which is oneof heat-generating electric components to be mounted later and is incontact with the metal plate (B). In Comparative Example 4, a ceramiccircuit board was fabricated by the same technique, without providingthe notch in the metal plate (A).

Next, as the heat sink was prepared a JIS named 1050 aluminum plate witha size of 60×140×4 mm having four tapped holes. A JIS named 2017 Al foilwas inserted between two of the above-mentioned ceramic circuit boardsand this aluminum plate, and the heat sink and the ceramic circuitboards were bonded with the application of pressure in the verticaldirection by use of a graphite jig, in a vacuum on the order of 10⁻³ Pawith the application of heat thereto 590° C. for 10 minutes, whereby amodule structure was fabricated.

Next, the changes in warp were measured with the assumption of atemperature history before and after the soldering of the semiconductordevice. The warp was measured by the following method. First, the shapeof the bottom of the module structure after the bonding was measured inthe longitudinal direction from one end to the other end (span: 140 mm)by use of a contour analyzer of a stylus type, and value determinationthereof was carried out with both-ends compensation. The modulestructure was then subjected to heat treatment at 360° C. for 10minutes, and the shape of the bottom was measured to determine thedifference in shape before and after heating. The maximum difference wasassumed as the amount of warp. The results are shown in Table 4.

Next, in order to evaluate the heat dissipation performance, the modulestructure was plated with nickel in the entire surface thereof byelectroless plating, and a 10 mm-square semiconductor device wassoldered onto a predetermined position of the circuit at 360° C. in areducing atmosphere by use of a high-temperature solder. Thecross-sectional structures of the modules of Example 12 and ComparativeExample 4 are shown respectively in FIGS. 1 and 2. An aluminum heatdissipation unit was fixed on the bottom face of the module structurewith four screws through silicone grease therebetween. The thermalresistance was determined by measuring the temperature of a silicondevice and the aluminum heat dissipation unit, while cooling the heatdissipation unit with water, and causing a constant current to flow inthe thickness direction of the silicon device. The results are shown inTable 4.

TABLE 4 Differential Thermal Position of notch warp* resistance* Ex. 12Metal plate (A) 50 100 Ex. 13 Portion of upper 38 100 face of heat sinkin contact with the ceramic circuit board Ex. 14 Portion of upper 60 100face of heat sink out of contact with the ceramic circuit board Ex. 15Lower face of heat 75 100 sink Comp. None 100 100 Ex. 4 Comp. Metalplate (A) 42 130 Ex. 5 *The values are relative ones when the value ofComparative Example 4 is 100.

The obtained module was subjected to 3,000 heat cycles with each cyclebeing −40° C.×30 minutes→room temperature×10 minutes→125° C.×30minutes→room temperature×10 minutes. Subsequently, the bonding interfacebetween the ceramic substrate and the heat sink was examined with anultrasonic flaw detector to determine the presence or absence ofdebonding at the bonding interface. In any modules, abnormalities suchas the debonding of the circuit board and the occurrence of cracks inthe silicon nitride substrate caused by the heat cycles were notobserved.

COMPARATIVE EXAMPLE 5

A module was fabricated in the same manner as in Example 12 except thata cutout to be introduced into the metal plate (A) was provided belowthe silicon device downward in the vertical direction. In the samemanner as in Example 12, the changes in the warp of the semiconductordevice before and after the soldering of the semiconductor device andthe thermal resistance were evaluated. The cross-sectional structure ofthe module is shown in FIG. 3, and the results of evaluation are shownin Table 4.

EXAMPLE 13

A module was fabricated in the same manner as in Comparative Example 4except that a groove with a width of 1 mm and a depth of 1.5 mm wasformed using a diamond cutter in such a region that is outside a frustumof cone region formed by a group of straight lines drawn downward at anangle of 45° with respect to the vertical direction from the edges ofthe semiconductor device to be mounted in a later step in contact withthe metal plate (B) and in a portion where the aluminum heat sink is incontact with the ceramic circuit board. The module was evaluated in thesame manner as in Comparative Example 4. The cross-sectional structureof the module is shown in FIG. 4, and the results of the evaluation areshown in Table 4.

EXAMPLES 14 AND 15

Modules, each comprising a ceramic circuit board bonded to an aluminumheat sink, were fabricated by the same method as in Comparative Example4. In Example 14, grooves were formed at the positions of the upper faceof the heat sink as shown in FIG. 5. In Example 15, grooves were formedat the positions of the lower face of the heat sink as shown in FIG. 6,each groove having a width of 3 mm and a depth of 2 mm. Thereafter,modules were fabricated and evaluated by the same method as inComparative Example 4. The results are shown in Table 4.

INDUSTRIAL APPLICABILITY

The module structure and the module using the same of the presentinvention are characterized in that despite the use of an inexpensivemetal in the heat sink, the deformation is small even under atemperature history at the mounting of a semiconductor device or thelike, the assembling thereof is easy, abnormalities such as thedebonding at the bonding interface, the fatigue breaking of the aluminumlayer, the cracking of the ceramic substrate, and cracks in the solderlayer, hardly occur, and the heat dissipation performance is excellent,so that the module structure and the module using the same of thepresent invention are suitable for power modules for variousapplications, in particular, to power modules for transportationapparatus, and therefore are industrially very useful.

1. A module structure comprising a ceramic circuit board bonded to a metal heat sink through a metal plate having a main component of which is aluminum, wherein the thickness of said metal plate is 400 μm or more to 1200 μm or less, wherein the heat sink comprises an aluminum alloy having a Vickers hardness of 30 Hv or more.
 2. The module structure according to claim 1, wherein said metal plate is bonded to the ceramic circuit board and to the metal heat sink through a brazing filler metal.
 3. The module structure according to claim 2, wherein the brazing filler metal comprises Al as the main component, Mg, and at least one element selected from the group consisting of Cu, Zn, Ge, Si, Sn, and Ag.
 4. A module structure comprising a ceramic circuit board bonded to a metal heat sink through a first metal plate having a main component of which is aluminum, wherein the thickness of said first metal plate is 400 μm or more to 1200 μm or less, further comprising: a heat-generating electric component provided at a predetermined position of another metal plate with a circuit formed thereon, the another metal plate being provided on the side of the ceramic circuit board opposite the first metal plate, and a notch provided at the surface of at least one of said first metal plate and said metal heat sink, wherein in a cross section of said module, said notch is provided in such a region that is outside a frustum of a cone formed by straight lines extending through the another metal plate and extending from edges of said heat-generating electric component in contact with the another metal plate at an angle of 45° with respect to a vertical direction.
 5. The module according to claim 4, wherein the notch is provided on the surface of the first metal plate in contact with the metal heat sink.
 6. The module according to claim 4, wherein the notch is provided on the surface of the metal heat sink covered with the ceramic substrate when viewed from the heat-generating electric component of said module.
 7. The module according to claim 4, wherein the notch is provided on the surface of the metal heat sink which is not covered with the ceramic substrate when viewed from the heat-generating electric component of said module. 